PREFACE 



This book has been written in order to achieve two objects, first to 

 place before the reader in as succinct a form as possible the best ascer- 

 tained results in the field of Invertebrate Embryology, and secondly 

 to indicate some of the problems which as yet remain unsolved and 

 the best means of attacking them. 



In order to attain the first object a number of typical life-histories, 

 illustrating all the important groups of Invertebrata, have been 

 described, and in selecting the types for special description two 

 principles have guided us : first, the life-history of the type chosen 

 must be thoroughly ascertained, and second, the type must be a 

 common form easily accessible to students in temperate regions. 

 Thus the spider has been chosen as a type of the Arachnida rather 

 than the scorpion, and for the same reasons the life-histories of 

 parasitic forms have been very slightly dealt with. The Trematoda 

 and Cestoda have been entirely left out of consideration because it 

 is diEScult to obtain a complete series of the stages in the life-history 

 of any one species — and though the external features of the life- 

 history of members of these groups are known, their organogeny is 

 still to be worked out. Moreover, the external features of the 

 development of Trematoda and Cestoda are adequately described in 

 ordinary text-books of zoology. 



In pursuit of the second object the methods used by the best 

 investigators have been given in connection with the description of 

 the life-history of each type examined by them, and we have ever 

 striven to keep before the mind of the student the idea that the 

 ultimate object of the Science of Embryology is not solely the 

 ascertaining of facts but especially the determination of the laws of 



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